TROVE-2017-13: Use-after-free in onion service v2 when rotating intro points
SEVERITY: High
ALSO TRACKED AS: CVE-2017-8823
DESCRIPTION
An onion service v2 expires its intro points regularly at least
once very 24 hours. While removing an intro point, if no circuit
is found, it is put in a retry list. Then just after, if it is
removed because it is expiring, it is put in the expiring list.
Tor then tries to open a circuit to that node and, on failure, it
will free the intro point without removing it from the expiring
list ultimately leading to a use-after-free.
This can only happens in specific conditions which are that the
service's is unable to launch circuits, this can happen if it is
missing descriptors for instance and if the intro points was just
being expired. It only affects version 2 services.
MITIGATION NOTES:
1. If you are not running an onion service, this doesn't affect
you.
2. If you are running tor version <= 0.2.6, this doesn't affect
you.
3. We believe this to be quite difficult to trigger remotely
because of the specific conditions that tor needs to be
in. However, it could be possible but hard to be induced by a
malicious Guard node suspecting a connection to be an onion
service.
ACKNOWLEDGMENTS:
Thanks to an anonymous reporter on our bugtracker that opened a
ticket which lead to the discovery of this issue.
FIX:
Anybody running an onion service on an affected version should
upgrade to one of the releases with the fix for this issue:
0.2.8.17, 0.2.9.14, 0.3.0.13, 0.3.1.9, or 0.3.2.6-alpha.